1. Field of the Invention
The present invention relates generally to the fields of medicine and biology. More particularly, it concerns compositions and methods for the treatment of cancer with enzymes that deplete both L-cystine and L-cysteine. Even more particularly, it concerns the engineering of a primate enzyme with high cysteine/cysteine degrading activity and stability suitable for human therapy.
2. Description of Related Art
Systemic depletion of various amino acids has been shown to be effective in killing a wide variety of tumor types with minimal toxicity to non-cancerous tissues. This therapeutic effect can be achieved through the use of pharmacologically optimized enzymes introduced into circulation that degrade the amino acid upon which the tumor relies. Certain cancers, such as prostate, small cell lung carcinomas, glioblastomas, and hepatocellular carcinomas, have been shown to be heavily dependent on extracellular cysteine/cystine in order to proliferate and survive. Many of these tumors aberrantly overexpress the xCT(−) cystine/glutamate antiporter in order to maintain sufficient cysteine levels needed for protein and glutathione production, suggesting that they have lost or down-regulated their native cysteine biosynthetic capacity. In support of this idea, the use of small molecule inhibitors of xCT(−) cystine/glutamate antiporters, such as sulfasalazine, have been shown to retard the growth of prostate and small cell lung cancer tumor xenografts (Doxsee et al., 2007; Guan et al., 2009). Although a blockade of the xCT(−) dependent transport of L-cystine is promising it does not eliminate the transport of free L-cysteine by the Na+-dependent ASC transporter and/or Na+-independent transporters, and in some examples free L-cysteine is provided to tumor cells by bone-marrow derived stromal cells (Zhang et al., 2012). A therapeutic that depletes both cystine and cysteine can thus completely deprive tumors of this essential metabolite. However, there are no known irreversible cysteine/cystine degrading enzymes with properties or activity suitable to be applied as a human therapeutic, and in general, enzymes frequently do not exist that can accomplish specific amino acid degradation, necessitating the engineering of desired activities from existing enzymes.